tokio-stream: implement FusedStream for various stream adaptors (#8096)

`MapWhile` is intentionally excluded because its current implementation
does not track when the closure returns `None` early, making a correct
`is_terminated()` impossible without a separate semantic change.

tokio-stream/src/stream_ext/skip_while.rs

@@ -3,6 +3,7 @@ use crate::Stream;
 use core::fmt;
 use core::pin::Pin;
 use core::task::{ready, Context, Poll};
+use futures_core::FusedStream;
 use pin_project_lite::pin_project;
 
 pin_project! {
@@ -71,3 +72,13 @@ where
         (lower, upper)
     }
 }
+
+impl<St, F> FusedStream for SkipWhile<St, F>
+where
+    St: FusedStream,
+    F: FnMut(&St::Item) -> bool,
+{
+    fn is_terminated(&self) -> bool {
+        self.stream.is_terminated()
+    }
+}